Yayun Wan

Adaptive Transthoracic Refocusing of Dual-Mode Ultrasound Arrays

We present experimental validation results of an adaptive, image-based refocusing algorithm of dual-mode ultrasound arrays (DMUAs) in the presence of strongly scattering objects. This study is motivated by the need to develop noninvasive techniques for therapeutic targeting of tumors seated in organs where the therapeutic beam is partially obstructed by the ribcage, e.g., liver and kidney. We have developed an algorithm that takes advantage of the imaging capabilities of DMUAs to identify the ribs and the intercostals within the path of the therapeutic beam to produce a specified power deposition at the target while minimizing the exposure at the rib locations. This image-based refocusing algorithm takes advantage of the inherent registration between the imaging and therapeutic coordinate systems of DMUAs in the estimation of array directivity vectors at the target and rib locations. These directivity vectors are then used in solving a constrained optimization problem allowing for adaptive refocusing, directing the acoustical energy through the intercostals, and avoiding the rib locations. The experimental validation study utilized a 1-MHz, 64-element DMUA in focusing through a block of tissue-mimicking phantom [0.5 dB/(cm?MHz)] with embedded Plexiglas ribs. Single transmit focus (STF) images obtained with the DMUA were used for image-guided selection of the critical and target points to be used for adaptive refocusing. Experimental results show that the echogenicity of the ribs in STF images provide feedback on the reduction of power deposition at rib locations. This was confirmed by direct comparison of measured temperature rise and integrated backscatter at the rib locations. Direct temperature measurements also confirm the improved power deposition at the target and the reduction in power deposition at the rib locations. Finally, we have compared the quality of the image-based adaptive refocusing algorithm with a phase-conjugation solution obtained by direct measurement of the complex pressures at the target location. It is shown that our adaptive refocusing algorithm achieves similar improvements in power deposition at the target while achieving larger reduction of power deposition at the rib locations.

Imaging with concave large-aperture therapeutic ultrasound arrays using conventional synthetic-aperture beamforming

Several dual-mode ultrasound array (DMUA) systems are being investigated for potential use in image- guided surgery. In therapeutic mode, DMUAs generate pulsed or continuous-wave (CW) high-intensity focused ultrasound (HIFU) beams capable of generating localized therapeutic effects within the focal volume. In imaging mode, pulse-echo data can be collected from the DMUA elements to obtain B-mode images or other forms of feedback on the state of the target tissue before, during, and after the application of the therapeutic HIFU beam. Therapeutic and technological constraints give rise to special characteristics of therapeutic arrays. Specifically, DMUAs have concave apertures with low f-number values and are typically coarsely sampled using directive elements. These characteristics necessitate pre- and post-beamforming signal processing of echo data to improve the spatial and contrast resolution and maximize the image uniformity within the imaging field of view (IxFOV). We have recently developed and experimentally validated beamforming algorithms for concave large-aperture DMUAs with directive elements. Experimental validation was performed using a 1 MHz, 64- element, concave spherical aperture with 100 mm radius of curvature. The aperture was sampled in the lateral direction using elongated elements 1 - lambda times 33.3macr - lambda with 1.333macr - lambda center-to-center spacing (lambda is the wavelength). This resulted in f-number values of 0.8 and 2 in the azimuth and elevation directions, respectively. In this paper, we present a new DMUA design approach based on different sampling of the shared concave aperture to improve image quality while maintaining therapeutic performance. A pulse-wave (PW) simulation model using a modified version of the Field II program is used in this study. The model is used in generating pulse-echo data for synthetic-aperture (SA) beamforming for forming images of a variety of targets, e.g., wire arrays and speckle-generating cyst phantoms. To provide validation for the simulation model and illustrate the improvements in image quality, we show SA images of similar targets using pulse-echo data acquired experimentally using our existing 64-element prototype. The PW simulation model is used to investigate the effect of transducer bandwidth as well as finer sampling of the concave DMUA aperture on the image quality. The results show that modest increases in the sampling density and transducer bandwidth result in significant improvement in spatial and contrast resolutions in addition to extending the DMUA IxFOV.

Imaging vascular mechanics using ultrasound: Phantom and in vivo results

We introduce a new method for simultaneous imaging of tissue motion and flow with subsample accuracy in both axial and lateral directions. The method utilizes a phase-coupled 2D speckle tracking approach, which employs the true 2D complex cross correlation to find subpixel displacements in both axial and lateral directions. We have also modified the imaging sequence on a Sonix RP scanner to allow high frame rate 2D data collection in a limited field of view covering the region of interest (M2D-mode). Together with the robust 2D speckle tracking method, M2D imaging allows for capturing the full dynamics of the flow and wall/tissue motion, even when the flow is primarily in the lateral direction (with respect to the imaging beam). The fine vector displacement estimates in both axial and lateral directions are shown to allow for smooth and contiguous strain and shear strain calculations with minimal filtering. The quality of the displacement and strain fields is demonstrated by experimental results from a flow phantom (ATS Model 524) and in vivo images of the carotid artery in a healthy volunteer. The results clearly demonstrate the feasibility of simultaneous imaging of the vector flow field and the wall/tissue motion and the corresponding strains at high spatial and temporal sampling. This may provide an essential tool in modeling the fluid-solid interactions between the blood and blood vessel, a key challenge in vascular biomechanics.

A post-beamforming 2-D pseudoinverse filter for coarsely sampled ultrasound arrays

Beamforming artifacts due to coarse discretization of imaging apertures represent a significant barrier against the use of array probes in high-frequency applications. Nyquist sampling of array apertures dictates center-to-center spacing of lambda/2 for elimination of grating lobes in the array pattern. However, this requirement is hard to achieve using current transducer technologies, even at the lower end of high-frequency ultrasonic imaging (in the range 25?35 MHz). In this paper, we present a new design approach for 2-D regularized pseudoinverse (PIO) filters suitable for restoring imaging contrast in systems employing coarsely sampled arrays. The approach is based on a discretized 2-D imaging model for linear arrays assuming scattering from a Cartesian grid in the imaging field of view (FOV). We show that the discretized imaging operator can be represented with a block Toeplitz matrix with the blocks themselves being Toeplitz. With sufficiently large grid size in the axial and lateral directions, it is possible to replace this Toeplitz-block block Toeplitz (TBBT) operator with its circulant-block block circulant (CBBC) equivalent. This leads to a computationally efficient implementation of the regularized pseudoinverse filtering approach using the 2-D fast Fourier transform (FFT). The derivation of the filtering equation is shown in detail and the regularization procedure is fully described. Using FIELD, we present simulation data to show the 2-D point-spread functions (PSFs) for imaging systems employing linear arrays with fine and coarse sampling of the imaging aperture. PSFs are also computed for a coarsely sampled array with different levels of regularization to demonstrate the tradeoff between contrast and spatial resolution. These results demonstrate the well-behaved nature of the PSF with the variation in a single regularization parameter. Specifically, the 6 dB axial and lateral dimensions of the PSF increase gradually with increasing value of the regularization parameter. On the other hand, the peak grating lobe level decreases gradually with increasing value of the regularization parameter. The results are supported by image reconstructions from a simulated cyst phantom obtained using finely and coarsely sampled apertures with and without the application of the regularized 2-D PIO.

Simultaneous imaging of tissue motion and flow velocity using 2D phase-coupled speckle tracking

We introduce a new method for simultaneous imaging of tissue motion and flow with subsample accuracy in both axial and lateral directions. The method utilizes a phase-coupled 2D speckle tracking approach, which employs the true 2D complex cross correlation to find subpixel displacements in both axial and lateral directions. We have also modified the imaging sequence on a Sonix RP scanner to allow high frame rate 2D data collection in a limited field of view covering the region of interest (M2D-mode). Together with the robust 2D speckle tracking method, M2D imaging allows for capturing the full dynamics of the flow and wall/tissue motion, even when the flow is primarily in the lateral direction (with respect to the imaging beam). The fine vector displacement estimates in both axial and lateral directions are shown to allow for smooth and contiguous strain and shear strain calculations with minimal filtering. The quality of the displacement and strain fields is demonstrated by experimental results from a flow phantom (ATS Model 524) and in vivo images of the carotid artery in a healthy volunteer. The results clearly demonstrate the feasibility of simultaneous imaging of the vector flow field and the wall/tissue motion and the corresponding strains at high spatial and temporal sampling. This may provide an essential tool in modeling the fluid-solid interactions between the blood and blood vessel, a key challenge in vascular biomechanics.

Quadratic B-mode and pulse inversion imaging of perfusion defects in vivo

In this paper, a comparison of quadratic B-mode (QB-mode) and pulse inversion (PI) imaging for characterization of perfusion defects in heat treated tumor model in vivo is presented. SCK mammary carcinomas grown in A/J mice were treated with 125 or 250 mug/kg CYT-6091 (formerly known as PT-cAu-TNF) followed by local heating at 42.5degC using a water-bath for 60 min, 4 hrs after nanoparticle injection. Ultrasound imaging was performed on days 1 and 5 after treatment using a modified Technos MPX system from ESAOTE S.p.A. (Genoa, Italy). A linear array probe (LA522E, 2-cycle transmit pulse centered at 5.5 MHz) was used to collect data in PI mode at 26 fps. The contrast agent, BR14 (Bracco Research S.A., Geneva, Switzerland) which is a new experimental agent consisting of high molecular weight perfluorobutane gas bubbles (2 mum diameter) encapsulated by a flexible phospholipids shell and suspended in saline was administered by the tail vein at a concentration of 0.0025 mL/kg of body weight. QB-mode images were obtained from the quadratic components from an adaptive second order Volterra filter (SoVF). A recursive least square (RLS) algorithm was used to estimate the quadratic kernel of the SoVF. PI images were formed in the usual manner by summing the echoes from the positive and negative transmit pulses. The RF echo data from the positive transmit pulse was used as input to the SoVF to extract the quadratic component. Results: PI images (typically 40 dB dynamic range) showed heterogeneous perfusion profile in the treated tumor 1 and 5 days after the treatment. This was quantified by computing a temporal perfusion index (TPI) for each image pixel from a sequence of 126 frames. QB-mode images (typically 70 dB dynamic range) showed similar heterogeneity in the perfusion pattern. However, the results illustrate an important potential advantage of this imaging mode. Specifically, QB-mode images have high dynamic range with reduced speckle with high sensitivity and specificity to nonlinear oscillations from UCA. Conclusion: QB-mode images have shown the same level of sensitivity to perfusion defects as PI images. QB-mode images do not require multiple pulse transmission thus they are not vulnerable to motion artifacts. In addition, they achieve higher specificity due to speckle reduction and noise suppression.

Image‐Based Refocusing of Dual‐Mode Ultrasound Arrays (DMUAs) in the Presence of Strongly Scattering Objects

An advantage of imaging with DMUAs is the potential for identifying target and critical regions in the treatment field for avoidance. Refocusing of the therapeutic beam in the presence of strongly scattering objects, such as the ribs, while targeting liver tumors is of particular importance due to limited access and distortion of the HIFU beam. An image‐based refocusing algorithm utilizing gray‐scaled images obtained with single‐transmit focus imaging allows for selection of control points to be taken from the visible target and ribs in the image. Using a two‐step virtual array method to take advantage of the intercostal spacing of the ribs, the algorithm minimizes the power deposition over the critical regions while maintaining or improving the power deposition at the target location. The algorithm is verified experimentally with a 64‐element 1MHz array, in an attenuating tissue mimicking phantom ( .5 dB/cm/MHz) with Plexiglas ribs. Thermocouples are used to measure sub‐therapeutic temperatures across th...

2C-1 A Post-Beamforming 2D Pseudoinverse Filter for Coarsely Sampled Ultrasound Arrays

Beamforming artifacts due to coarse discretization of imaging apertures represent a significant barrier toward the use of array probes in high frequency ultrasound (HFUS) applications. In this paper, we present a new design approach for two-dimensional (2D) regularized Pseudoinverse filters suitable for restoring imaging contrast in systems employing coarsely sampled arrays. The approach is based on a discretized 2D imaging model for linear arrays, and the imaging operator can be represented with a block Toeplitz matrix with the blocks themselves being Toeplitz. With sufficiently large grid size in the axial and lateral directions, it is possible to replace this Toeplitz-block block Toeplitz (TBBT) operator with its circulant-block block circulant (CBBC) equivalent. This leads to a computationally- efficient implementation of the regularized Pseudoinverse filtering approach using the 2D fast Fourier transform (FFT). Using FIELD II, we present simulation data of the 2D point spread functions (PSFs) and a cyst phantom for imaging systems employing linear arrays with fine and coarse sampling of the imaging aperture. The 6-dB axial and lateral dimensions of the PSF, the main-lobe grating-lobe ratio (MGR) and contrast ratio (CR) are computed for the coarsely-sampled array with different levels of regularization to demonstrate the trade off between contrast and spatial resolutions. These results demonstrate approximate sigmoid functions with respect to log10beta which reveals a smooth, well-behaved regularization process of this algorithm.

Dereverberation of ultrasound echo data in vascular imaging applications

Two-dimensional speckle tracking methods have shown great promise in imaging tissue motion and deformations in the vicinity of blood vessels offering the promise of new methods for detecting and staging of vascular disease. However, vessel wall echo reverberations overwhelm the echoes (scattering) from the blood and result in loss of flow information in large regions within the vessel. In this paper, we present a design approach for a time-varying dereverberation inverse filter for echo data within the vessel. The design approach is motivated by the fact that the reverberation pattern varies significantly with the pulsatory motion of the vessel wall. Minute changes in the location/orientation of the vessel wall with respect to the imaging beam result in measurable changes in the speckle-specular echo mixture at the vessel wall and the observed periodicities in the reverberation pattern within the vessel. Therefore, a time-varying inverse filter is necessary to remove the reverberation components appropriately during the heart cycle. A maximum likelihood approach for optimizing the inverse filter parameter is presented. The performance of the dereverberation algorithm is illustrated using pulse-echo data from realtime imaging of the carotid artery of a healthy volunteer.

2D filter design for the reduction of beamforming artifacts in coarsely-sampled imaging apertures

We have previously introduced a k-space based 2D post-beamforming filter for restoring image contrast loss due to coarse sampling of the imaging apertures. The 2D regularized imaging operator was obtained and implemented on full frame data in k-space using computationally-efficient 2D fast Fourier transform (FFT). The computational efficiency of the 2D filter can be further improved by implementing the filter in the spatial domain provided the filter has finite region of support (ROS). We have developed a weighting algorithm for limiting the ROS to approximately 0.03% of the full k-space implementation. The algorithm was verified using Field II in imaging a speckle-generating cyst phantom. We simulated two 25 MHz linear arrays with lambda/2 and with 2lambda element spacing. The simulation results verify that the 2D spatial filter with finite ROS can achieve the same level of restoration in CR while maintaining the same level of spatial resolution achieved by the full k-space filtering approach. We also present the first experimental verification of the performance of our 2D PIO approach on linear array imaging of quality assurance phantom with contrast targets. RF frame data from cystic targets were collected and processed using an appropriately designed 2D PIO. The results show that the CR values were increased by approximately 8 dB without significant reduction in spatial resolution.